Joshua 4:3

3 And command ye them, saying, Take you hence out of the midst of Jordan, out of the place where the priests' feet stood firm, twelve stones, and ye shall carry them over with you, and leave them in the lodging place, where ye shall lodge this night.

twelve men were told to take stones from the middle of the Jordan River, what did that signify? To be a permanent memorial of how the flow of the Jordan River stopped as the Ark of the Covenant passed through.